Mobile communication systems such as cellular phone systems are now widely used. Meanwhile, in order to increase the speed and capacity of radio communication, there have been lively discussions on the next-generation mobile communication technology. For example, a standard called Long Term Evolution (LTE) has been proposed by the international standardization organization 3rd Generation Partnership Project (3GPP) (see, for example, 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation”, 3GPP TS 36.211 V9.1.0, 2010-03., 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, 3GPP TS 36.213 V9.1.0, 2010-03. and 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification”, 3GPP TS 36.331 V9.2.0, 2010-03.). Further, a standard called Long Term Evolution-Advanced (LTE-A) as an evolution of LTE has also been proposed (see, for example, 3rd Generation Partnership Project, “Feasibility study for Further Advancements for E-UTRA”, 3GPP TR 36.912 V9.0.0, 2009-09.).
In mobile communication systems, a relay station is often provided so as to relay communication between a base station and a mobile station. The provision of the relay station may expand the cell area and improve the throughput. However, in the relay station, interference (self-interference) might occur between a reception signal and a transmission signal of the relay station. For instance, in the case where the frequency bandwidth used for communication between the base station and the relay station overlaps the frequency bandwidth used for communication between the relay station and the mobile station, a radio signal transmitted to the mobile station might be input to a reception circuit of the relay station, so that a radio signal from the base station might not be successfully received. To solve this problem, there has been proposed a technique that prevents self-interference by adjusting the timing of communicating with a base station and the timing of communicating with a mobile station (see, for example, Section 9 of 3rd Generation Partnership Project, “Feasibility study for Further Advancements for E-UTRA”, 3GPP TR 36.912 V9.0.0, 2009-09.).
In a mobile communication system in which a base station and a mobile station communicate with each other via a relay station, a relay station may perform a handover so as to switch the destination base station. For example, in the case where a relay station is mounted on a vehicle such as a train and an automobile, if a mobile station is carried by a passenger in the vehicle and performs communication via a relay station, although a handover for switching the destination of the mobile station does not occur, a handover for switching the destination of the relay station may occur.
In this case, the handover by the relay station affects communication of the mobile station, which becomes a problem. For example, in some cases, the transmission timing of frames and symbols may vary between base stations. When the relay station performs a handover, transmission timing of frames and symbols from the relay station to the mobile station may be changed in accordance with the destination base station. The relay station that is to perform a handover may detect in advance a transmission timing of a base station to be applied after the handover, and may prepare for synchronization before the handover. On the other hand, when the handover is performed by the relay station, the mobile station connected to the relay station may suddenly become out of synchronization. When the mobile station determines that the mobile station is out of synchronization with the relay station, the mobile station may again perform a process of establishing synchronization, such as cell search, for example. Thus, the time lag taken by the mobile station to resume communication becomes a problem.